1. Field of the Invention
The present invention disclosed in the specification relates to a method of fabricating a thin film transistor using a crystalline semiconductor film.
2. Description of Related Art
Conventionally, there has been known a thin film transistor (hereinafter, referred to as TFT) using an amorphous silicon film. The transistor is utilized mainly for constituting an active matrix circuit of a liquid crystal display device of an active matrix type.
However, according to TFT using an amorphous silicon film, there poses a problem where the operational speed is retarded and a P-channel type one cannot be reduced to practice.
The transistor cannot be used in a liquid crystal display device of an active matrix type integrated with a peripheral drive circuit and various integrated circuits cannot be constituted by using such TFT because of such a problem.
There has been known a constitution using a crystalline silicon film as means for resolving the problem.
As methods of fabricating a crystalline silicon film, there are classified roughly into a method by heating and a method by irradiation of laser beam.
According to the method by heating, there poses a problem where a glass substrate cannot be utilized since a process at a high temperature as high as 900° C. or higher is needed.
In consideration of the fact that a major field of application of TFT is a liquid crystal display device, capability of utilizing a glass substrate as a substrate constitutes a problem with priority.
Meanwhile, according to the method by irradiation of laser beam, although a process in which a substrate does not undergo thermal damage can be realized, the process is not satisfactory in view of uniformity and reproducibility of crystallinity and a degree of crystallinity of the semiconductor film.
As a means for resolving such a problem, there has been a method of accelerating crystallization by using a predetermined catalyst element which is the invention of the applicant.
According to the method, a catalyst element represented by nickel is introduced into an amorphous silicon film and a crystalline silicon film is provided later by a heating treatment.
According to the method, a crystalline silicon film having excellent crystallinity can be provided by a heating treatment at about 600° C. or lower in which a glass substrate can be utilized.
However, nickel element remains in the crystalline silicon film by which adverse influence is effected on properties of TFT fabricated thereby.
Specifically, there poses a problem of aging change of the properties, deterioration in reliability or the like.